Many known types of dredging apparatus as well as anti-siltation systems utilize bodies incorporating venturi structures with a compressed gas or air introduced into the venturi to aid in "lifting" fluid and debris from the ocean floor or in the case of silt deposits to pick up the silt and redistribute it through the water.
As an example of the foregoing, reference is had to U.S. Pat. No. 3,855,367 to Webb which, while relating to a venturi type anti-siltation system, could, in a broad sense, be used for dredging. In the Webb structure, the diffuser portion of the venturi is fairly shallow and the walls form a relatively large angle with the vertical. The compressed gas or air for providing "lift" is introduced midway in this diffuser portion of the venturi and while the action is sufficient for anti-siltation problems in most instances, the arrangement illustrated is not well suited for deep ocean dredging operations. More particularly, it is found that a "sudden release" of the liquid sucked up through the venturi occurs at the diffuser portion and this "sudden release" is not desirable because it inhibits the flow of incoming fluid. If this diffuser portion of the venturi in the Webb patent were flared outwardly at a very gradual angle, no sudden release of the incoming fluid would occur and such would be an ideal situation were it not for the fact that the incoming air would flow directly into the fluid and thus experience some pressure resistance.
U.S. Pat. No. 2,361,861 issued to Masowich is concerned with a venturi arrangement for the removal of welding fumes and while this particular application is far afield from deep sea dredging operations, the structure disclosed merits careful consideration since many of the air lift principles are utilized. In the Masowich patent, the compressed air or gas is introduced through a continuous annulus at the throat of the venturi section through a plenum chamber. This type of continuous circular entry pattern results in the formation of large bubbles in the liquid passing up through the venturi. Such large bubbles tend to slip upwardly through the liquid and do not reduce the overall pressure of the liquid sufficiently. While the continuous open circular pattern for the compressed gas or air is not an appreciable problem where welding fumes are to be drawn upwardly, such a structure could introduce problems in deep sea dredging operations. For example, it would be difficult to empty the plenum chamber of any excess liquid if the air or gas flowed into the venturi pipe in a continuous circular pattern. If the apparatus vibrates or rocks back and forth while in use, the excess liquid would restrict at various alternating points the air or gas from flowing into the vertical pipe forming the continuation of the venturi.
From the foregoing, it can be appreciated that while basic venturi structures with the introduction of compressed gas or air are known, there still is a need for an improved structure of the air lift type suitable for deep sea dredging operations which can operate more efficiently than available equipment.